As mentioned before, a cheap alternative to buying textbooks online is renting them. For most people, the idea of renting a textbook is completely foreign but it can save you up to half the cost of a new book. The site which I use to rent most of my textbooks is coursesmart.com
Now I know a lot of people struggle to read books on the computer but it is well worth the savings. The first time I rented a textbook, it took me twice as long to read through it. But it didn't take long for me to adapt and now I can read through an eTextbook as quickly as I can a normal one.
So how does it work? You just go on coursemart and enter in the ISBN to see if they have your book or not. You pay for a 180 day subscription to the book and you can choose to access it on the web (so you can use over multiple computers) or download it as a pdf file (but can only be used on 1 computer). I usually choose to access it on the web because it is 2010 and there is internet access wherever I go and they have a neat highlight function in their browser reader.
The downside of renting is that you don't get a physical copy to sell after the semester. But come on now, how many of you actually sell them yourselves and not take the lazy route of getting pennies of a dollar by selling to your bookstore during buybacks?